For the recent drastic communication demand increase, wavelength division multiplex (WDM) communication systems which multiplex a plurality of light signals of different wavelengths to thereby transmit a large capacity of information by one optical fiber are being developed. Such WDM communication systems utilize a number of optical parts for multiplexing and demultiplexing signal light, and signal light is attenuated due to light losses in the respective optical parts.
To compensate such light losses, optical amplifying devices are used. The WDM communication system requires a much larger number of optical amplifying devices than the conventional optical fiber communication system. The optical amplifying devices used in the WDM communication system must be small-sized and can operate at small power consumption.
Furthermore, such optical amplifying devices must have large input dynamic ranges so as to meet large power level changes of input light signals and must have a function of controlling levels/powers of output light signals to be constant.
Among various light amplifiers, semiconductor optical amplifiers (SOAs) are small-sized and have low power consumption, and are expected to be optical amplifiers for the light loss compensation, which are used in the WDM communication systems.
The inventor of the present application has proposed an optical amplifying device using an SOA having a function of controlling a level/power of output signal light to be constant by injecting external light (refer to Patent Reference 1). The optical amplifying device using the conventional SOA having the function of controlling a level/power of output signal light to be constant by injecting external light will be explained with reference to FIG. 13. FIG. 13 is a block diagram of the conventional optical amplifying device having the function of controlling a light output level to be constant, which shows a structure thereof.
A light source for control light 102 is connected to one input port of a wavelength division multiplexing (WDM) coupler 100 which multiplexes signal light and control light. Signal light to be amplified is inputted to the other input port of the WDM coupler 100. An SOA 104 which amplifies signal light multiplexed with control light by the WDM coupler 100 is connected to the output port of the WDM coupler 100. An optical filter 106 which separates amplified signal light from control light is disposed on the output side of the SOA 104.
In addition, an optical divider 108 for branching a part of the signal light outputted from the optical filter 106 is disposed on the output side of the optical filter 106. A power meter 110 which measures the output of the branched signal light is connected to one output port of the optical divider 108. The power meter 110 is connected to a control unit 112 for controlling a light level of control light, based on the measured result of the output of the signal light.
Based on the measured result of an output of the signal light given by the power meter 110, a light level of control light which is to be coupled with the signal light to be inputted to the SOA 104 is controlled, whereby the amplification factor of the signal light to be amplified by the SOA 104 can be controlled.
However, discrete modules, in which the optical input/output is achieved by using optical fibers, are used for the respective constituent elements of the conventional optical amplifying device shown in FIG. 13. Each module is as large as several centimeters, and an about 10-cm square space is required for the whole optical amplifying device. Accordingly, its size is a disadvantage in using the optical amplifying device of the WDM communication system.
The conventional optical amplifying device shown in FIG. 13 includes a number of lenses, isolators and Peltier devices, which disadvantageously results in high costs. That is, generally, the light source for control light 102 requires 2 lenses, 1 isolator and 1 Peltier device; the SOA 104 requires 4 lenses, 2 isolators and 1 Peltier device; the WDM coupler 100 requires 2 lenses; and the filter requires 2 lenses. The optical amplifying device as a whole requires 10 lenses, 3 isolators and 2 Peltier devices. Thus, the conventional optical amplifying device having the function of controlling light output levels has a large number of the required optical parts, which makes the optical amplifying device expensive.
Furthermore, the step of mounting the modules constituting the optical amplifying device takes much work, which is one factor for high costs.
An object of the present invention is to provide an optical amplifying device having the function of controlling light output levels, which is small-sized, includes a small number of necessary optical parts and does not require a complicated mounting step.
Patent Reference 1
Japanese Patent Application Unexamined Publication No. 2000-208758